1. Field of Invention
This invention relates to sound movies, particularly to a method of providing the audio portion of such movies.
2. Prior-Art: Analog Optical Soundtrack Recording
The most common prior-art method for producing sound movies, i.e., for the storage of audio information on motion picture film, is the well known "variable-area" optical method. In this method, an elongated soundtrack runs down one side of the filmstrip, between one set of sprocket holes and the picture frames. This soundtrack commonly consists of an optically-translucent (or opaque) region whose width varies according to the instantaneous amplitude of the sound. During movie projection, the soundtrack is illuminated from behind with a lamp and lens and then viewed through a slit by a photocell. Due to the varying width of the transparent area of the soundtrack, the slit receives varying width illumination as the film moves. This causes the amount of light which reaches the photocell to vary correspondingly. The voltage which appears at the output of the photocell will thereupon vary linearly with the amount of light falling on its surface so that the output voltage of the photocell will represent the original audio. However it is usually not a faithful reproduction of original sound because of distortion created by, e.g., dirt on the filmstrip, wear of the soundtrack on the filmstrip, etc. The distortion usually takes the form of audible pops, scratches, rumbles, hisses, loss of frequency range, etc.
3. Prior-Art Magnetic Recording Schemes
Sound movies have also been made by applying a coating similar to that used on magnetic recording tape onto the motion picture filmstrip. This was unsuccessful because of technological difficulties. The magnetic medium did not adhere well to the motion picture film, resulting in a very short film life. In addition, because of the stiffness of the filmstrip, a high pressure was required to hold the magnetic track against the magnetic playback head. As a result, the track's recording medium, which contained abrasive iron oxide, eroded the playback heads to the point of uselessness after only a few showings of a film. This recording method also suffered from the usual noise sources present in magnetic tape recording.
4. Prior-Art Digital Recording Schemes
Russell, in U.S. Pat. No. 3,806,643, has taught another recording system. In Russell's system, analog electrical signals are converted to digital form and then stored on photographic film plates. He employs two data formats: one is serial (Russell, FIGS. 5, 6), as used in compact audio discs. In this case, the binary digital bits comprising a digital word are stored on a disc in a spiral pattern of ever-decreasing radius, much like the groove on a phonograph record. When the disc is rotated, the bits are read out seriatim. The other data format (Russell, FIGS. 7, 9) consists of blocks of data whose words are stored in parallel fashion. During readout, the entire filmplate is moved in a stepwise fashion so that one block of data at a time is located within the physical range of operation of a conventional beam-scanning apparatus. Then one row of data at a time is focused onto a photocell array and a horizontal row of bits, which comprise part or all of a word, or multiple digital words, is illuminated and focused onto a photocell array for electronic readout. In one case (Russell, FIG. 7) the bits in a row are illuminated seriatim. In another case (Russell, FIG. 9) an entire block of data is illuminated at once and one row at a time of said block of data is focused on a photocell array for seriatim readout.
While Russell's system is suitable for storage of large quantities of digital information, it has a number of practical disadvantages. It does not teach any way to provide the audio portion of a sound movie. Also, Russell requires one photocell per optical data bit. This can result in ambiguous, and thus false readouts of the digital data. Russell's system would thus be especially unsuitable for use with moving picture filmstrips because the filmstrip is continually moving during both recording and playback of the digitized audio information. Thus during such recording and playback, the film may wander from side to side. To require physical alignment between the physically separate recording and playback stations to a tolerance of less than one optical bit across the width of the column of bits would be impractical since each and every optical bit must be read out faithfully. It also requires precise scaling and positioning of the film plate and the data recording and playback components. Russell's means for scanning light beams are cumbersome and expensive. Further, Russell's requirement for stepwise motion of the film plates in the parallel data case is cumbersome.
Potter, in U.S. Pat. No. 2,595,701, has taught yet another digital recording and playback system. FIG. 1 of Potter shows a recording system in which the audio information is sampled and each sample is digitized to an amplitude resolution of eight bits. The eight-bit words representative of the amplitude of the respective samples are then converted to pulse-code modulation signals. Next, Potter uses a cumbersome arrangement employing a cathode-ray tube to cause a row of flashlamps to be fired in sequence starting at one end of the array and proceeding to the other end, instead of firing them simultaneously. Some lamps will fire and others will not in order to represent digital ONES and ZEROES. The flashlamps are imaged onto a moving photographic film strip. Ninety-six bulbs comprising twelve groups of eight flashlamps each are imaged across the width of the photographic film strip. The film in Potter's camera is moving continuously during recording. The motor which moves the film is driven at a constant speed by his data encoding system which also creates timing marks, one for each data row, through the use of an extra flashlamp whose image appears at one edge of the film. Since the bulbs are flashed serially while the filmstrip moves, the images of the bulbs will appear in sloped lines on the filmstrip. Potter proposes that the sloped rows be spaced apart by an amount which is 20 percent greater than the spacing between optical bits within a row in order to ensure that each optical bit will be read only once. This inefficient data packing reduces the amount of audio information which can be stored on a given area of filmstrip, resulting in a lower audio frequency response, reduced audio dynamic range, or both.
FIG. 5 of Potter shows a playback system. As the film is moved through the playback projector, a narrow framing slit causes the individual images of the flashlamps, which appear in sloped rows across the filmstrip, to be projected seriatim on a row of photocells. Again, Potter uses a cumbersome method employing a cathode ray tube to scan the photocell outputs seriatim in order to detect the presence or absence of optical data bits only when they are expected to be imaged on each photocell. The regular appearance of the synchronizing flashlamp image is used to control the speed of the motor which drives the filmstrip and to initiate each readout scan of the photocells. The combination of circuitry which detects the synchronizing spots, controls the motor speed and initiates the horizontal photocell scanning constitutes a closed-loop servomechanism for speed control. Such servomechanisms are cumbersome and expensive. Potter's system could not be easily or inexpensively adapted for use in standard motion picture equipment.
Again, as in the case of Russell, Potter uses one photocell per optical data bit, thus requiring precise registration of the optical data bit images and photocells. This results in the imposition of impractical mechanical tolerances when the data bit images must be made as small as the filmstrip resolution permits.